A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet radiation (EUV) having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that radiation with a wavelength of less than 10 nm could be used, for example 6.7 nm or 6.8 nm. In the context of lithography, wavelengths of less than 10 nm are sometimes referred to as ‘beyond EUV’.
Extreme ultraviolet radiation and beyond EUV radiation may be produced using a plasma. The plasma may be created for example by directing a laser at particles of a suitable material (e.g. tin), or by directing a laser at a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits extreme ultraviolet radiation (or beyond EUV radiation), which is collected using a collector, which receives the extreme ultraviolet radiation and focuses the radiation into a beam.
EUV sources, such as those which generate EUV radiation using a plasma, not only emit desired ‘in-band’ EUV radiation, but may also emit undesirable ‘out-of-band’ radiation. This out-of-band radiation is most notably in the deep ultra violet (DUV) radiation range (100-400 nm). Moreover, in the case of some EUV sources, for example laser produced plasma EUV sources, the radiation from the laser, usually at 10.6 μm, presents a significant amount of out-of-band radiation.
In a lithographic apparatus, spectral purity is desired for several reasons. One reason is that resist is sensitive to out-of-band wavelengths of radiation, and thus the image quality of patterns applied to the resist may be deteriorated if the resist is exposed to such out-of-band radiation. Furthermore, out-of-band radiation infrared radiation, for example the 10.6 μm radiation in some laser produced plasma sources, may lead to unwanted and unnecessary heating of the patterning device, substrate and optics within the lithographic apparatus. Such heating may lead to damage of these elements, a degradation in their lifetime, and/or defects or distortions in patterns projected onto and applied to a resist-coated substrate.
In order to overcome these potential problems, several transmissive spectral purity filters have been proposed which substantially prevent the transmission of infrared radiation, for example, radiation having a wavelength of 10.6 μm, while simultaneously allowing the transmission of EUV radiation. Proposed spectral purity filters may comprise a thin metal layer, and/or an array of apertures. These spectral purity filters have an EUV transmittance of typically 50-80%, with, for example, maximum values of around 78% for a Zr/Si multilayer foil.
In any lithographic apparatus, it will be appreciated that it is desirable to minimize the losses in intensity of radiation which is being used to apply a pattern to a resist coated substrate. One reason for this is that, ideally, as much radiation as possible should be available for applying a pattern to a substrate, for instance to reduce the exposure time and increase throughput.